Hexagonal ferrites: a unified model of the (TS)(n)T series in superspace

摘要

Hexagonal ferrites represent an extensive family of mixedlayer magnetic materials with periods up to 1500 A along the stacking direction, probably constituting the largest unit cells in the inorganic realm. The (TS)(n)T subfamily includes P (3) over bar m1 and R (3) over barm structures that can be derived from Y ferrite Ba2M2Fe12O22 ( M = Zn, Fe, Co, Mg, Mn) by introducing stacking faults. A unified ( 3 + 1)-dimensional superspace model is proposed for all members of the ( TS) nT family. The model belongs to the superspace group X (3) over bar m1(00 gamma) with X = {(1/3, 2/3, 0, 1/3), (2/3, 1/3, 0, 2/3)}, has a unit cell of the basic structure with a = 5.88, c = 4.84 angstrom and modulation vector [GRAPHICS] q = 4n+3/ 9n+6 c*, where n is rational for periodic structures and irrational for the aperiodic ones. The model was tested on calculated data of one of the principal members of the (TS)(n)T family, the Y ferrite ( n = infinity). The fit obtained with the superspace model was excellent. The model allowed a reduction of refinable parameters by 19% with respect to the ordinary refinement without a significant increase of the refinement R values.